Limits for Twist

Twist is the result of applied torsion on circular or square-sectioned shafts. If the twist disappears as a result of removing the force, then the shaft will have been loaded below its elastic limit. If the shaft remains twisted after removal of the load, then it has been loaded above its elastic limit.

The action of a shaft carrying a torque load is to twist in proportion to the torque applied. The result of cyclic loading of shafts is that the shafts have to be checked for permanent twist. If the shaft has a square section, it can be checked for twist on a surface table using a DTI mounted on a surface gauge.

Solid or tubular shafts that have to be checked for twist and will possibly have witness marks or lines engraved at each end of the shaft. The shafts can be checked by mounting the shaft in Vee blocks and then locating these marks in the horizontal position.

It is possible to measure the amount of twist, to which a shaft is subjected, whilst in operation or rotation, by the use of strain gauges. These emit varying amounts of electric current when under strain, giving an indication of the load being applied.

The designer of the aircraft or equipment will set all limits with regards to the distortion of parts and set them down in the relevant manuals. The methods used to measure the distortion will either be standard procedures such as using a DTI and surface table etc., or a special procedure included in the manuals.